It’s the only other planet in our solar system that we know is tectonically active.

Mercury has been hiding an exciting secret: the closest planet to our Sun is perpetually shrinking, according to new data from NASA, which shows the appearance of new, tiny cracks on Mercury's surface called "fault scarps". This means that out of all the planets in our solar system, only Mercury and the Earth are tectonically active, with crusts that change their shapes from the inside out.

The findings come from images collected by NASA's MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) spacecraft, and are scheduled to be published in the October issue of Nature Geoscience. The uncrewed MESSENGER orbited Mercury at an extremely low-altitude for part of its mission, allowing it to capture details of the planet's surface from closer than ever before. The images were obtained in the last 18 months of the MESSENGER's orbit, before the spacecraft crashed on Mercury on April 30, 2015.

"Mercury is not a body where most of the activity has occurred in the distant past," lead author and Smithsonian senior scientist Thomas Watters told Motherboard. "These faults are so small, they've got to have formed very recently." Because Mercury, like Earth, is tectonically active, the planet's contractions caused the faults to form.

"Mercury is shrinking slowly because its interior is hot and is slowly cooling off. As it cools down, the interior volume changes and contracts," said Watters. The resulting small fault scarps are like saplings on a tree thought to be extinct, he added.

The lower white arrows in this photo shows fault scarps on Mercury. Image: NASA/JHUAPL/Carnegie Institution of Washington/USGS/Arizona State University

Large fault scarps, which appear like rocky cliffs, were first found on Mercury in the mid-1970s and confirmed that the planet had contracted in the past. While the largest of Mercury's faults are comparable in size to the San Andreas, these newly found tiny fault scarps are only ten meters tall and a few kilometers wide. And since Mercury doesn't have a protective atmosphere like Earth's, these fault scarps must be young enough to have survived consistent meteoroid bombardment. They're comparable in scale to the small scarps found on the moon, which is also shrinking and tectonically active.

If those scarps had been bombarded a million years ago, scientists wouldn't be able to see them now, said Watters. Observing Mercury's small scarps has been an "interesting development," he said, because it shows that the planet hasn't cooled down yet—contrary to what its small size would suggest. "Because it's so small, it should have cooled down billions of years ago, but it turns out that's just not the case," said Watters. "Mercury has retained its interior heat."

This means that smaller bodies in the solar system don't behave the way scientists thought, he added. The assumption that small planets like Mercury become "geologically inactive very quickly" has been proven false. "The big implication is that maybe we don't understand how the terrestrial planets have evolved," said Watters. ("Terrestrial" planets like Mercury, Earth, Venus, and Mars are rocky, as compared with gaseous planets like Jupiter, Saturn, Uranus, and Neptune in the outer solar system.)

When the solar system and the planets formed 4.6 billion years ago, the Earth and Mercury grew to be very different sizes. Mercury's diameter is about 3,000 miles, while Earth's is 8,000. While the new findings don't suggest that Mercury could sustain life (its temperature extremes, between 800 and -800 degrees fahrenheit are too extreme), it could have other characteristics similar to Earth.

"I think it's going to help us understand in the broader sense this range of possibilities Earth-like planets could have by better understanding the Earth and terrestrial like planets in our solar system," said Watters. "[This] gives us more more insight into the many many extrasolar planets being discovered, especially those that are Earth-like."